Electrodes for discharge light sources, such as cathodes for cold cathode discharge, are usually made of Ni, Ni-Fe, Ni-Cr-Cu, or the like, which have a good efficiency with respect to secondary emission, so that low firing potential and high emission luminance are achieved.
The cathode for cold cathode discharge is now required to have a higher performance with respect to secondary emission as discharge displays come into practical use.
To meet this requirement, it has become a common practice to make the cathode for cold cathode discharge from a material composed of a metal conductor and an alloy or metal oxide laminated thereon, which has a high efficiency with respect to secondary emission
Examples of the alloy or metal oxide (for secondary emission) to be laminated on the metal conductor include BaAl (described in the Institute of Electronics and Communication Engineers of Japan, ED81-25, pp. 61-66, 1981, Sakai et.al.), LaB6 (described in the research paper of the Institute of Telecommunications Engineers, ED-572, pp. 55-60, 1982, Kamegaya et.al.), and MgO (disclosed in Proc. SID, 22/4, pp. 219-227, 1981, M. O. Aboeifoth).
The conventional electrode for discharge light source is shown in section in FIG. 4. The electrode is attached to the discharge container 1 enclosing a rare gas alone or a rare gas containing mercury vapor which is necessary for discharging. The discharge container 1 also functions as a glass substrate.
The inside wall of the discharge container 1 is coated with a fluorescent material 2, and the substrate of the discharge container 1 is provided with a metal conductor 3.
The metal conductor 3 is made of Al or Ni, and its surface has a deposited film 5 of secondary emission material.
The film 5 of secondary emission material forms a cathode for a discharge light source.
The function is described in the following. The application of a voltage (necessary for discharging) to the metal conductor 3 gives kinetic energy to initial electrons which have already been generated by photoionization in the discharge container 1, causing them to ionize the rare gas or the mercury vapor-containing rare gas, thereby generating ions and electrons. The thus generated initial ions collide with the film 5 of the material for secondary emission (which functions as the cathode) to generate secondary electrons. These electrons move to the anode, and each time when they collide with neutral atoms during their move, they generate electrons or excited atoms in geometrical progression, permitting the self-maintaining discharge. It follows, therefore, that the smoother and more large the generation of initial electrons, the less the energy to be injected for discharging (initial firing potential) and the energy to be injected for maintaining discharge (discharge maintaining voltage). In other words, it is possible to realize a highly efficient discharge light source or discharge display by employing the film 5 of the material for secondary emission which efficiently emits secondary electrons when hit by ions.
In the meantime, it is known that the secondary emission caused by ion collision occurs according to the Auger neutralization mechanism and the Auger de-excitation mechanism (as described by H. D. Hagstrum in Phys, Rev., 96(2), pp. 336-365, 1954. It is also known that these mechanisms permit more secondary emission as the solid metal decreases in work function .PHI. (ease of transition from Fermi level to valence band) and Fermi level .delta..sub.F (Fermi energy measured from the bottom of the conduction band).
For this reason, efforts are being made to reduce the work function of the material for secondary emission in order to lower the firing potential and to increase the efficiency.
Table 1 shows the work function (eV) of typical materials for secondary emission, which was taken from J. Chem. Phys. 60 (10), pp. 4076-4080, 1974, S. Yamamoto et. al.
TABLE 1 ______________________________________ Ni Fe Ba MgO LaB.sub.6 ______________________________________ 4.96 4.49 2.51 4.20 3.20 ______________________________________
The conventional electrode for discharge light source is constructed as mentioned above, and, in practice, it is made of La.sub.6 B or MgO having a low work function as the material for secondary emission These materials, however are not necessarily satisfactory in the rate of secondary emission, and this prevents the apparatus from being improved in performance such as higher brightness and lower discharge voltage.
A conceivable way of coping with this is to use Ba, which has a higher rate of secondary emission than LaB.sub.6 and MgO. Unfortunately, Ba in the form of simple substance is so chemically active that it reacts with the constituent materials such as metal conductor 3 and fluorescent material 2, reducing the life of the apparatus, and it readily reacts with moisture and oxygen in air to form BaO while the apparatus is being produced Therefore, Ba does not provide stable performance Disclosure of the invention:
It is an object of the present invention to provide an electrode for discharge light source having stable performance and high reliability in order to lower the firing potential, increase the energy conversion efficiency, and raise the brightness, while keeping the high rate of secondary emission, without reducing the life of the apparatus and causing such troubles as the formation of BaO in the course of apparatus production.
This object is achieved by an electrode for discharge light source, which is characterized in that the metal conductor formed in the discharge container in which a rare gas is enclosed is 1-5 .mu.m thick, and the film of the material for secondary emission formed on the metal conductor is made of a compound composed of LaB and Ba in an amount of 0.01--20 mol % of LaB.sub.6 and is 0.5-2 .mu.m thick.
The above described electrode for discharge light source has the following features The metal conductor having a thickness limited to 1-5 .mu.m withstands the discharge current of tens of mA. The film of the material for secondary emission which is formed on the metal conductor from a compound composed of LaB.sub.6 and Ba in an amount of 0.01-20 mol % of LaB.sub.6 provides the electrode for discharge light source which has good heat resistance, chemical stability, and superior secondary emission characteristics. In addition, the film of the material for secondary emission which has a thickness limited to 0.5-2 .mu.m exhibits its performance fully, without suffering from pin-holes.
In a second embodiment, the electrode for discharge light is characterized in that the film of the material for secondary emission is made of a compound composed of LaB.sub.6, Ba in an amount of 0.01-20 mol % of LaB.sub.6, and Ca in an amount of 0.01-5 mol % of Ba, and is 0.5-2 .mu.m thick.
The second embodiment of the electrode for discharge light sources has the following features. The film of the material for secondary emission which is formed from a compound composed of LaB.sub.6, Ba in an amount of 0.01-20 mol % of LaB.sub.6, and Ca in an amount of 0.01-5 mol % of Ba and is 0.5-2 .mu.m thick provides the electrode which is chemically stable over a long period of time and easy to handle and has an extremely high rate of secondary emission. This lowers the firing potential and increases the energy conversion efficiency.